This disclosure generally relates to free-space laser communication systems. More particularly, this disclosure relates to free-space atmospheric laser communication systems having error control circuitry.
Telemetry is an automated communications process by which measurements are made and other data collected at remote or inaccessible points and transmitted to receiving equipment for monitoring. For example, it is known to acquire telemetry data onboard an aircraft and then transmit that data to a receiver on the ground in response to telecommands from the ground. One existing solution uses a radio-frequency (RF) link to transmit telemetry data between an airplane and a ground station. The drawbacks of using an RF link include a low data rate and vulnerability to electromagnetic interference.
Free-space atmospheric laser optical communication systems transmit and receive information by means of light beams that propagate through the atmosphere. The benefits of using a free-space optical communication link instead of the existing RF link include at least the following: higher-speed telemetry data transmission and the free-space optical signal is not affected by electromagnetic interference.
Free-space laser optical communication is a viable solution for a long-distance (>1 km) telemetry link between an aircraft and a ground station because the technologies embodied in high-data-rate transmitters, high-power optical amplifiers and high-sensitivity receivers are mature and commercially available. However, there are three major problems for optical transmission of telemetry data: (1) Free-space laser communication has an atmospheric turbulence effect which causes optical beam wander between the transmitter and receiver locations. (2) Optical signals in free space experience a fading effect which is caused by destructive interference of reflected signals from other objects in the free space to the receiver. As a result of destructive interference, the optical signal at the receiver experiences loss of signal bits or generates erroneous signal bits in a digital optical data stream. (3) Without a standard for free-space high-speed optical communication, popular Gigabit Ethernet (GBE) protocol is preferable for high-speed telemetry data. This may cause an incompatibility problem with existing laser communication equipment designed for a high-speed Synchronous Optical Network (SONET) protocol. Problems (1) and (2) are inherent problems of free-space optical communication; problem (3) is an optical system component design issue.
The present disclosure is directed toward free-space laser communication systems capable of addressing one or more of the aforementioned problems.